A boosted engine may exhibit higher combustion and exhaust temperatures than a naturally aspirated engine of similar output power. Such higher temperatures may cause increased nitrogen-oxide (NOX) emissions from the engine and may accelerate materials ageing, including exhaust-aftertreatment catalyst ageing. Exhaust-gas recirculation (EGR) is one approach for combating these effects. EGR works by diluting the intake air charge with exhaust gas, thereby reducing its oxygen content. When the resulting air-exhaust mixture is used in place of ordinary air to support combustion in the engine, lower combustion and exhaust temperatures result. EGR may also improve fuel economy in gasoline engines by reducing throttling losses and heat rejection.
In boosted engine systems equipped with a turbocharger compressor mechanically coupled to a turbine, exhaust gas may be recirculated through a high pressure (HP) EGR loop or through a low-pressure (LP) EGR loop. In the HP EGR loop, the exhaust gas is taken from upstream of the turbine and is mixed with the intake air downstream of the compressor. In an LP EGR loop, the exhaust gas is taken from downstream of the turbine and is mixed with the intake air upstream of the compressor.
HP and LP EGR strategies achieve optimum efficacy in different regions of the engine load-speed map. For example, on boosted gasoline engines running stoichiometric air-to-fuel ratios, HP EGR is desirable at low loads, where intake vacuum provides ample flow potential; LP EGR is desirable at higher loads, where the LP EGR loop provides the greater flow potential. Various other tradeoffs between the two strategies exist as well, both for gasoline and diesel engines. Moreover, each strategy presents its own control-system challenges. For example, HP EGR is most effective at low loads, where intake vacuum provides ample flow potential. At higher loads, it may be difficult to maintain the desired EGR flow rate. On the other hand, LP EGR provides adequate flow from mid to high engine loads, but may respond sluggishly to changing engine load, engine speed, or intake air flow. In gasoline engines especially, such unsatisfactory transient response may include combustion instability during TIP-out conditions, when fresh air is needed to sustain combustion but EGR-diluted air is present upstream of the throttle valve. Opening a compressor by-pass valve at this time provides a partial, but incomplete remedy for the problem, as EGR-diluted air remains upstream of the throttle, albeit at a lower absolute pressure. Moreover, a significant lag in EGR availability can occur during TIP-in conditions, as the amount of EGR accumulated in the intake manifold may not be sufficient to provide the desired combustion and/or emissions-control performance.
Various approaches have targeted transient control issues in engine systems equipped for EGR. For example, U.S. Pat. No. 6,470,682 to Gray, Jr. provides a base intake manifold through which fresh air and cooled LP EGR are provided to a diesel engine, and, an additional intake manifold that supplies only fresh air to the engine. The additional intake manifold is sourced by a fast-acting, electrically driven air compressor. When torque demand increases rapidly, the fast-acting compressor is switched on, displacing the existing mixture of air and EGR in the base intake manifold and providing increased oxygen mass to the engine, for increased torque. However, this system is particular to diesel engines, which may be unthrottled, and may tolerate significant amounts of EGR even at idle. Accordingly, the particular transient-control issues addressed in Gray, Jr. differ from those experienced in spark-ignition engines.
The inventors herein have recognized that improved transient control in an LP EGR equipped engine system can be achieved with the aid of a unique port-mounted throttle. In one embodiment, therefore, an engine system is provided. The engine system comprises an air cleaner, a combustion chamber coupled to an intake port, and an intake manifold. The intake manifold is configured to receive air from the air cleaner, and, under some conditions to receive exhaust from the combustion chamber. The engine system further comprises a multifunction, barrel-type throttle valve coupled to the intake port via an outlet, the throttle valve having a first inlet coupled to the intake manifold and a second inlet coupled to the air cleaner.
Among various other advantages, the throttle valve enables the engine system to rapidly switch between inducting an EGR-containing air charge from the intake manifold and inducting fresh air from the air cleaner. This approach effectively addresses at least some of the transient control difficulties of EGR-equipped engine systems.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.